In the third book of his Ecclesiastical History (written around 325), Eusebius says that in the earliest days the Church “remained a pure and uncorrupted virgin, for those who attempted to corrupt the healthful rule of the Savior’s preaching, if they existed at all, lurked in obscure darkness.” It wasn’t long, though, before enemies—internal and external—made themselves known.

“When the sacred band of the apostles and the generation of those to whom it had been vouchsafed to...